Micromachined apparatus with split vibratory masses

ABSTRACT

Each of a number of resonator masses is split into two separate lobes or masses joined together by a short flexure. The short flexure allows the separate lobes or masses to rotate slightly as they resonate so as to substantially relieve longitudinal stresses in certain resonator structures.

PRIORITY

This application is a divisional of U.S. patent application Ser. No.11/065,878 entitled MICROMACHINED SENSOR WITH QUADRATURE SUPPRESSIONfiled Feb. 25, 2005 now U.S. Pat. No. 7,032,451, which is a divisionalof U.S. patent application Ser. No. 10/360,987 entitled MICROMACHINEDGYROSCOPE filed Feb. 6, 2003, now U.S. Pat. No. 6,877,374, which claimspriority from U.S. Provisional Patent Application No. 60/354,610entitled MICROMACHINED GYROSCOPE filed Feb. 6, 2002 and U.S. ProvisionalPatent Application No. 60/364,322 entitled MICROMACHINED GYROSCOPE filedMar. 14, 2002. The above-referenced patent applications are herebyincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to micromachined gyroscopes, andmore particularly to micromachined gyroscopes that use Coriolisacceleration to detect rotational movement.

BACKGROUND OF THE INVENTION

Micromachined structures, particularly gyroscopes, often require a masssuspended by flexures to move a distance which is large compared withthe flexure width. Then, as well as the lateral movement, the flexure isalso stretched longitudinally which, if unrelieved, makes the stiffnessmuch larger. Such a structure generally has an ill-defined resonantfrequency and is therefore unsuitable for making accurate measurements.Longitudinal tension can be relieved by transverse flexures at the endsof the primary flexures or by folding the primary flexures, as shown inU.S. Pat. No. 5,349,855. These solutions are not suitable when it isnecessary to stiffly constrain the mass in the longitudinal direction.Then, pairs of pivoted levers can be used as in U.S. Pat. No. 6,505,511.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide both longitudinal stressrelief to the flexures and longitudinal restraint to the mass withoutthe need of separate levers by making the mass in two portions joined bya very short flexure, which allows them to pivot without separatingsensibly.

In one embodiment of the invention there is provided apparatuscomprising a plurality of resonator structures including a pair of splitmasses suspended by suspension flexures, wherein each split massincludes two lobes interconnected with a flexure that allows the lobesto rotate slightly as they resonate so as to reduce longitudinalstresses in the suspension flexures.

In another embodiment of the invention there is provided apparatuscomprising a plurality of resonator structures including a first pair ofmasses coupled through a first flexure and a second pair of massescoupled through a second flexure, each mass suspended by a suspensionflexure, wherein the first and second flexures allow the masses torotate slightly as they resonate so as to reduce longitudinal stressesin the suspension flexures.

In related embodiments, the lobes/masses may be interconnected through aplurality of levers so as to resonate in anti-phase with one another, inwhich case the rotation of the lobes/masses may reduce longitudinalstresses in the levers. The plurality of levers typically transform thecoupled motion of the lobes from co-linear motion to parallel motion.Each lobe/mass may include a plurality of drive fingers interdigitatedwith a corresponding array of fixed drive fingers affixed to asubstrate. Each lobe/mass may include at least one notch for electronicquadrature suppression. The plurality of resonator structures may besuspended within an inner perimeter of a frame and the resonatorstructures may be mechanically coupled to produce substantially a singleresonance frequency so as to restrict transfer of inertial forces to theframe. Each of the plurality of levers may be coupled at one end to theframe and at another end to a different one of the lobes/masses, andeach lever may have pivots, defined at the points of attachment to theframe and the lobe/mass by the intersection of the axes of at least twoorthogonal flexures, to ensure that the attachment point cannottranslate with respect to the lever. Each of the levers may include aplurality of lever fingers interdigitated with corresponding fingersaffixed to an underlying substrate for at least one of driving the leverand sensing position of the lever. Each of the levers may move with anarcuate motion, and the lever fingers may be disposed at varying anglesso as to maintain substantially equal distances from said correspondingfixed fingers during movement of the levers. The plurality of resonatorstructures may be micromachined from a single wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 shows an exemplary micromachined gyroscope structure inaccordance with an embodiment of the present invention;

FIG. 2 identifies various components of the micromachined gyroscopestructure in accordance with an embodiment of the present invention;

FIG. 3 shows a highlighted view of the frame of the micromachinedgyroscope structure in accordance with an embodiment of the presentinvention;

FIG. 4 shows a highlighted view of a movable mass of the micromachinedgyroscope structure in accordance with an embodiment of the presentinvention;

FIG. 5 shows a highlighted view of a lever of the micromachinedgyroscope structure in accordance with an embodiment of the presentinvention;

FIG. 6 shows a detailed view of an accelerometer suspension flexure inaccordance with an embodiment of the present invention;

FIG. 7 shows a detailed view of a movable mass and its related flexuresand pivot flexures in accordance with an embodiment of the presentinvention;

FIG. 8 shows a detailed view of two levers and a fork and their relatedpivot flexures and electrostatic driver in accordance with an embodimentof the present invention;

FIG. 9 shows a representation of the motions of the various resonatingstructures of the micromachined gyroscope structure in accordance withan embodiment of the present invention;

FIG. 10 shows the coriolis detector switch-overs for the doubledifferential configuration in accordance with an embodiment of thepresent invention;

FIG. 11 shows a detailed view of an electrostatic driver for a movablemass in accordance with an embodiment of the present invention;

FIG. 12 shows a detailed view of quadrature suppression structures inaccordance with an embodiment of the present invention;

FIG. 13 shows an alternate frame suspension configuration in accordancewith an embodiment of the present invention; and

FIG. 14 shows a configuration for drive or sensing fingers that areincorporated into a coupling level in which the fingers are raked backat varying angles to accommodate the arcuate motion of the couplinglever, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

A micromachined gyroscope includes a resonator made to oscillate with avelocity and an accelerometer means for measuring the orthogonalCoriolis acceleration which results from the effect of rotation on thatvelocity. The usual means of attaching these structures to each otherand to an underlying substrate is by filaments of micromachined materialthat are often referred to as “tethers” or “flexures.”

Thus, a micromachined gyroscope makes use of Coriolis acceleration todetect and measure rotation rate about an axis normal to the surface ofa substrate. Specifically, various resonating structures are suspendedwithin a frame. The resonating structures include phase and anti-phasemasses that are mechanically coupled through levers, pivot flexures, andforks in order to produce a single resonance frequency for the entireresonating system. The mechanical system ensures that the motion of theresonators is severely restricted to one linear axis with no netrotation. Rotation of the micromachined gyroscope about the planeproduces a rotational force on the frame. The frame is suspended in sucha way that its motion is severely restricted in all but the rotationaldirection. Sensors on all sides of the frame detect the rotationaldeflection of the frame for measuring the change in direction.

It has been recognized in the prior art of micromachined gyroscopes thatbalanced (or symmetric) structures give significantly better performanceand that mechanically coupled pairs of resonators are much to bedesired. See, for examples, U.S. Pat. Nos. 5,392,650 and 5,635,638.Mechanically coupling the resonating structures has a number ofadvantages, including increasing the motion of the resonatingstructures, increasing the amount of Coriolis acceleration (signal)produced by the resonating structures, avoids chaotic motion, preventsmotion of the frame in the same direction as the resonating structures,provides better phase definition, provides better rejection of externalaccelerations, and improves the quality factor Q because angularmomentum is canceled locally.

It has proven possible to make satisfactory gyroscopes if a pair ofresonators is coupled by electrical means only. An example is describedin “Single-Chip Surface Micromachined Integrated Gyroscope” (IEEE JSSCvol. 37 No. 12 December '02) and U.S. Pat. No. 6,122,961. Manufacturingtolerances are such that two mechanically separate resonators cannot befabricated with identical frequencies but if the “Q” factors are lowenough their resonance curves overlap sufficiently for the pair tofunction smoothly as a single electrical oscillator.

If more such devices could be manufactured per silicon wafer then thecost of each would be less, so there is an advantage in making smallerstructures. In order to obtain low noise and adequate signal fromsmaller structures, it is necessary to design their resonances withhigher “Q” factors. Then, the resonance curves may no longer overlapadequately, making the oscillation lower in amplitude and ill defined infrequency. In extreme cases, the motion becomes chaotic with verydeleterious effects on low frequency noise (i.e. short term outputinstability which prevents accurate navigation, one of the primary usesof the gyroscope).

Certain embodiments of the present invention allow smaller structures tobe used by providing a very effective mechanical coupling based on a“double fork” as described in U.S. Pat. No. 5,635,640 for rotatingresonators. Co-linear resonator pairs cannot be directly coupled in thisway so a set of levers is used to transform the coupled motion fromco-linear to parallel motion. The levers have pivots defined at thepoints of attachment to the accelerometer frame and resonator mass. Eachpivot point is defined by the intersection of the axes of at least twoorthogonal flexures. This ensures that the attachment point cannottranslate with respect to the lever, only rotate. Translationalcompliance would compromise suppression of unwanted motions, asdescribed later. Netzer displays a similar idea in FIGS. 8 through 11 ofU.S. Pat. No. 5,763,781. However, none of those structures will work ina practical micromachined gyroscope of the type described in thisdisclosure because, first, the pivots are defined by single flexuresallowing unacceptable orthogonal motions, second, the same defect ofdesign allows too much compliance to in-phase motion of the coupledmasses and, third, they allow no stress stiffening relief, the provisionof which is essential, as also described later.

It is also known to be very advantageous to suppress the so-called“quadrature” signals which arise from the resonator and accelerometeraxes being imperfectly orthogonal. The suppression means may beelectrical, as described by Howe et al in U.S. Pat. Nos. 6,067,858 and6,250,156, or mechanical as described by Geen in U.S. Pat. No.6,122,961. The latter uses separate resonator and accelerometer framestogether with a system of levers and flexures to inhibit unwantedmotions and is very effective in practice. However, that configurationis topologically incompatible with direct mechanical coupling of theresonators. First, half the accelerometer fingers, those in between theresonators, would be lost, reducing the signal substantially. Second,the linear Coriolis forces from the resonator pair would cancel in anaccelerometer frame attached to them both.

Certain embodiments of the present invention permit the mechanicalcoupling of resonators without loss of accelerometer signal from aseparate, quadrature-suppressed frame. This is accomplished byrecognizing that the coupled antiphase resonator masses produce aCoriolis torque proportional to the separation of their centers of masseven though the linear Coriolis forces cancel. Thus, a surroundingaccelerometer frame can be adapted to detect rotational rather thanlinear motion. Then, mechanical quadrature suppression becomes a matterof inhibiting any net rotational motion of the co-linear resonator pairand preventing linear motions of the accelerometer. Also, all four sidesof a rectangular accelerometer frame move when it rotates so that allmay be lined with fingers to detect that motion, thereby restoring thetotal sensitivity to that of two linear accelerometers but in half thetotal area compared with the prior art of U.S. Pat. No. 6,122,961.

Another problem encountered is that for a large Coriolis signal theresonators should have a large travel. The primary flexures of theresonator will “stress stiffen” in these circumstances. That is, theyhave to reach further when deflected and the resulting stretching causeslongitudinal tension in the flexure with a marked increase in lateralstiffness. The relative increase in stiffness is well known to vary asthe square of the ratio of the lateral deflection to the width of thetether. Thus, a typical 1.7 micrometer wide tether deflecting by 10micrometers would stiffen by a factor of 36 which would giveunacceptable non-linearity, require much more drive force and make theresonant frequency ill-defined by a large factor. This longitudinalstress can be relieved by simple transverse flexures, as in U.S. Pat.No. 5,392,650, but this allows overall resonator motion in the tetherlongitudinal direction and prevents mechanical quadrature suppression.Longitudinal stress can also be relieved by folding the primaryflexures, as shown in U.S. Pat. No. 5,349,855. These are not suitablewhen it is necessary to stiffly constrain the mass in the longitudinaldirection. Then, pairs of pivoted levers can be used as in U.S. Pat. No.6,505,511.

Certain embodiments of the present invention provide a means ofrelieving the longitudinal tension in the tethers without taking thespace for extra levers as were used in the prior art of U.S. Pat. No.6,122,961. This is achieved by using the resonator masses themselves,including the drive mechanism, as stress reduction levers. Theelimination of the extra, tether-suspended levers not only saves area,but also enhances the overall out-of-plane stiffness of the gyroscope.This makes the device more rugged and suitable for use in vehiclelocations experiencing large shocks or vibration, such as in the enginecompartment of a car.

In certain embodiments of the invention, a resonator mass is modified torelieve tension by splitting it and rejoining with a very short flexure.This allows the mass to pivot slightly about the flexure such thatdiagonally opposite corners can then simultaneously accommodate theshortening of the projected lengths of both the primary resonatorflexure and the coupling lever. The distances from the pivot to theflexure and lever must be in the correct proportion to effectivelyrelieve both so the positioning of the short flexure is critical, butthis not a difficult calculation in geometry.

In an exemplary embodiment of the present invention, the micromachinedgyroscope includes two phase masses and two anti-phase masses that aremechanically coupled through levers, pivot flexures, and forks. Ideally,this provides a single resonance frequency. The single resonancefrequency provides a higher Q factor, and therefore more signal. Theconfiguration of the coupling reduces extraneous forces on the frame(such as translational and rotational forces caused by unbalanced motionof the resonating structures) that can be misread as Coriolisaccelerations.

Certain embodiments of the present invention incorporate drive orsensing fingers into the coupling levers in order to save area. Theeffectiveness of the resonator mass in producing Coriolis torque fromits velocity is proportional to its distance from the center line.Consequently, it is desirable that resonator drive apparatus, orvelocity sensing apparatus for the purpose of completing anelectromechanical oscillator, should be placed as close to the centerline as possible. This most effectively utilizes the available area.Removing part of the apparatus from the mass to the coupling lever is,therefore, particularly advantageous.

Because the lever moves in an arc, interdigitated fingers placed on itmesh at different angles along its length, depending on the radius fromthe pivot point of the lever. Therefore, in order to prevent the movingfingers from excessive lateral motion with respect to a fixed,interdigitated comb, the fingers may be raked back at varying angles asdictated by the geometry of the lever. This is shown in FIG. 14.

A similar issue exists with the accelerometer frame and the sensingfingers placed around its periphery, since the accelerometer frame isadapted to rotate. Therefore, the sensing finger could likewise be rakedback at varying angles. However, the rotation of the accelerometer frameis typically much less than the rotation of the levers (perhaps1/100,000^(th)), so such raking is typically not used for the sensingfingers.

Certain embodiments of the present invention allow the quadrature signalto be finely trimmed to near null. Despite the suppression of quadratureby the configuration of the suspension flexures and levers, there is aresidual quadrature element from distortion of the accelerometer frameby the reaction forces of the resonator suspended from it. It isdesirable to keep this frame as light as possible both to save space andto maximize its response to Coriolis forces. Unfortunately, a lightframe distorts more, so there is a compromise in design which allowssome residual quadrature. This can be trimmed to near zero using ageneral principle described by Clark in U.S. Pat. No. 5,992,233 whichuses an array of fingers arranged in groups of 3 at different voltagesso as to provide a lateral force which varies with the meshing of thefingers. Embodiments of the present invention instead use a notch cutfrom the edge of the resonator mass. This has the advantage of consumingless space than the finger array and lending itself to beingaccommodated in otherwise unusable areas.

The drive fingers work longitudinally using interdigitated combs, somemoving, and some attached to the substrate. The principle is thatdescribed by Tang and Howe in U.S. Pat. No. 5,025,346. One of the mosttroublesome side effects of using longitudinal electrostatic comb drivesfor gyroscopes is that small imbalances of the gaps between the fingersinduce lateral motion as well as the desired longitudinal component.This motion has a component with the unfortunate property of beingin-phase with the Coriolis signal so that, unlike the much largerquadrature signal, it cannot be rejected by a phase sensitive rectifier.Any instability of this in-phase signal becomes directly a gyroscopeerror. One of the most significant ways in which the gaps can becomeimbalanced is by relative motion of the substrate anchor points of thefixed fingers and the moving structure. Another is the displacement ofthe moving structure from external accelerations. Fortunately, most ofthese can be made to cancel by careful attention to the symmetries ofthe structure and the drive apparatus. However, surface shear distortionof the substrate is particularly difficult to accommodate in this way.It is easily caused by variations in package stress induced during useand produces both a relative displacement of arrays of fixed fingers anda rotation of the individual finger anchors.

In certain embodiments of the present invention, the anchors for pairsof antiphase arrays of fixed drive fingers are arranged to be co-linearin the lateral direction. In this way, any surface shear of thesubstrate will not cause them to move laterally with respect to eachother. Also, the anchors are typically laid down in pairs joined to eachother at the top ends, remote from the substrate, so that the topsresist the individual twisting at the substrate end. Furthermore, thefinger busbars are typically attached to the top ends by flexible,folded fingers. These provide isolation of the busbar from anydistortion transmitted by the anchor pairs and from displacement byshrinkage stresses in the micromachined material. They also serve asdrivers thereby minimizing the loss of drive from the isolationmeasures. The finger attachment means provide about an order ofmagnitude improvement in the gyroscope performance.

FIG. 1 shows an exemplary micromachined gyroscope structure 100 inaccordance with an embodiment of the present invention. Micromachinedgyroscope structure 100 is typically one of many micromachined from asingle silicon wafer. The micromachined gyroscope structure 100 istypically mounted to a substrate. The micromachined gyroscope structure100 is substantially symmetrical top-to-bottom along the x axis as wellas side-to-side along the y axis.

FIG. 2 identifies various components of the micromachined gyroscopestructure 100. Among other things, the micromachined gyroscope structure100 includes a substantially square frame 210 that is suspended at itsfour corners by accelerometer suspension flexures 202, 204, 206, and208. FIG. 3 shows the frame 210 highlighted. On the outside four edgesof the frame 210 are fingers 212, 213, 214, 215, 216, 217, 218, and 219.Various resonating structures are suspended within the frame 210. Theseresonating structures include four movable masses 220, 222, 224, and226, four levers 228, 230, 232, and 234, and two forks 236 and 238. FIG.4 shows the mass 220 highlighted. It should be noted that the masses222, 224, and 226 are substantially the same shape, size, and mass asthe mass 220, and are oriented as mirror images of the mass 220 alongthe x and/or y axes. FIG. 5 shows the lever 228 highlighted. It shouldbe noted that the levers 230, 232, and 234 are substantially the sameshape, size, and mass as the lever 228, and are oriented as mirrorimages of the lever 228 along the x and/or y axes. The four movablemasses 220, 222, 224, and 226 are suspended from the frame 210 byflexures 240, 242, 244, and 246, respectively. Movement of the fourmovable masses 220, 222, 224, and 226 is controlled electrostaticallyusing electrostatic drivers 248, 250, 252, 254, 256, 258, 260, and 262.These and other features of the micromachined gyroscope structure 100are described in more detail below.

The four accelerometer suspension flexures 202, 204, 206, and 208 helpto control movement of the frame 210 relative to the substrate. The fouraccelerometer suspension flexures 202, 204, 206, and 208 substantiallyrestrict movement of the frame 210 along the x axis and along the y axis(i.e., translational movement), but allow the frame 210 to rotate morefreely in either direction (i.e., rotational movement). Such rotationalmovement of the frame 110 is caused mainly from the coriolis effect dueto movement of the frame of reference of the resonating structures.

FIG. 6 shows the accelerometer suspension flexure 202 in greater detail.The accelerometer suspension flexure 202 is anchored to the substrate atlocations 630 and 640. The accelerometer suspension flexure 202substantially restricts translational movement of the frame 210, butallows for rotational movement of the frame 210. The structures 650 and660 are etch equalizers that are used to ensure accurate formation ofthe other flexure structures. This principle is taught in U.S. Pat. No.6,282,960. It should be noted that the accelerometer suspension flexures204, 206, and 208 are substantially the same as the accelerometersuspension flexure 202.

The fingers 212, 213, 214, 215, 216, 217, 218, and 219 extend from thefour sides of the frame 210. Positioned between the fingers 212, 213,214, 215, 216, 217, 218, and 219 are two sets of coriolis detectors.

FIG. 6 shows the relationship between a finger 212 and two coriolisdetectors 610 and 620.

The two sets of coriolis detectors 610 and 620 are mechanically coupledto the substrate and do not move relative to the substrate. Movement ofthe frame 210 results in movement of the fingers 212, 213, 214, 215,216, 217, 218, and 219 relative to the coriolis detectors, as describedbelow. Movement of the fingers 212, 213, 214, 215, 216, 217, 218, and219 relative to the coriolis detectors produces a change in capacitancethat can be measured by electronic circuitry (not shown). This can bedone in a variety of ways.

The two sets of coriolis detectors 610 and 620 are coupled through fourswitch-overs 1010, 1020, 1030, and 1040 in a double differentialfashion, as shown in FIG. 10. The switch-overs 1010, 1020, 1030, and1040 substantially cancel signals induced electrically from surroundingcircuits and signals produced by translational movement of the frame 210but substantially amplify signals produced by rotational movement of theframe 210. Specifically, when there is translational movement of theframe 210, approximately half of the coriolis detectors produce a signaland the other half produce a substantially equal and opposite signal,resulting in a net signal of zero. Thus, translational movements of theframe 210 are substantially canceled out electronically. When there isrotational movement of the frame 210, however, all coriolis detectorsproduce complementary signals that, when combined and amplified,represents the magnitude of the rotational movement. By placing fingersand coriolis detectors on all sides of the frame 210, a larger signal isproduced as opposed to a solution in which fingers and coriolisdetectors are placed on only two sides of the frame 210.

The resonating structures, including the masses 220, 222, 224, and 226,the flexures 240, 242, 244, and 246, the levers 228, 230, 232, and 234,and the forks 236 and 238, are mechanically coupled. With referenceagain to FIG. 2, the masses 220 and 222 are mechanically coupled via apivot flexure 264, and the masses 224 and 226 are mechanically coupledvia a pivot flexure 266. The masses 220 and 224 are mechanically coupledvia the levers 228 and 230 and the fork 236, and the masses 222 and 226are mechanically coupled via the levers 232 and 234 and the fork 238.The pivot flexures 264 and 266, the levers 228, 230, 232, and 234, andthe forks 236 and 238 allow the masses 220, 222, 224, and 226 to movetogether.

The mass 220 is suspended from the frame 210 by the flexure 240, fromthe mass 222 by the pivot flexure 264, and from the lever 228 by thepivot flexure 268. The mass 222 is suspended from the frame 210 by theflexure 242, from the mass 220 by the pivot flexure 264, and from thelever 232 by the pivot flexure 272. The mass 224 is suspended from theframe 210 by the flexure 244, from the mass 226 by the pivot flexure266, and from the lever 230 by the pivot flexure 276. The mass 226 issuspended from the frame 210 by the flexure 246, from the mass 224 bythe pivot flexure 266, and from the lever 234 by the pivot flexure 280.

The lever 228 is suspended from the frame 210 by the pivot flexure 270,from the mass 220 by the pivot flexure 268, and from the lever 230 bythe fork 236. The lever 230 is suspended from the frame 210 by the pivotflexure 278, from the mass 224 by the pivot flexure 276, and from thelever 228 by the fork 236. The lever 232 is suspended from the frame 210by the pivot flexure 274, from the mass 222 by the pivot flexure 272,and from the lever 234 by the fork 238. The lever 234 is suspended fromthe frame 210 by the pivot flexure 282, from the mass 226 by the pivotflexure 280, and from the lever 232 by the fork 238.

FIG. 7 shows the mass 220 and related components in greater detail. Themass 220 is suspended from the frame 210 by the flexure 240, from themass 222 by the pivot flexure 264, and from the lever 228 by a pivotflexure 268. The flexure 240 is preferably formed from three paralleletches, where the center etch is unbroken and the outer etches arebroken in two places. The outer etches are etch equalizers that are usedto ensure accurate formation of the center etch. It should be noted thatthe masses 222, 224, and 226 and their related components aresubstantially the same as the mass 220 and its related components.

FIG. 8 shows the levers 228 and 230 and their related components ingreater detail. The lever 228 is suspended from the frame 210 by thepivot flexure 270, from the mass 220 by the pivot flexure 268, and fromthe fork 236 by the pivot flexure 820. The lever 230 is suspended fromthe frame 210 by the pivot flexure 278, from the mass 224 by the pivotflexure 276, and from the fork 236 by the pivot flexure 830. The fork236 is suspended from the lever 228 by the pivot flexure 820 and fromthe level 230 by the pivot flexure 830. It should be noted that thelevers 232 and 234 and their related components are substantially thesame as the levers 228 and 230 and their related components.

The flexures 240, 242, 244, and 246 substantially restrict movement ofthe masses 220, 222, 224, and 226 respectively along the y axis, butallow movement of the masses 220, 222, 224, and 226 respectively alongthe x axis. The flexures 240, 242, 244, and 246 also allow the masses220, 222, 224, and 226 respectively to pivot slightly as they move.

The pivot flexure 264 essentially locks the masses 220 and 222 togetherso that they move together. Likewise, the pivot flexure 266 essentiallylocks the masses 224 and 226 together so that they move together(although oppositely to the masses 220 and 222).

The levers 228 and 230, the fork 236, and the pivot flexures 268, 270,820, 830, 276, and 278 essentially lock the masses 220 and 224 togetherso that they move in substantially equal but opposite directions. Thelevers 232 and 234, the fork 238, the pivot flexures 272, 274, 280, and282, and the pivot flexures coupling the levers 232 and 234 to the fork238 (not shown) essentially lock the masses 222 and 226 together so thatthey move in substantially equal but opposite directions.

The levers 228 and 230 essentially translate the substantially equal butopposite side-to-side motion of the masses 220 and 224 into asubstantially linear motion of the fork 236 along the y axis.Specifically, the side-to-side motion of the mass 220 is transferred tothe lever 228 through the pivot flexure 268, while the side-to-sidemotion of the mass 224 is transferred to the lever 230 through the pivotflexure 276. The levers 228 and 230 pivot at pivot flexures 270 and 278,respectively, and at pivot flexures 820 and 830, respectively, to causethe linear motion of the fork 236 along the y axis. These transferscause the masses 220 and 224 to rotate slightly as they moveside-to-side. Specifically, the mass 220 rotates slightly toward themass 222 when moving to the left and slightly away from the mass 222when moving to the right, while the mass 224 rotates slightly toward themass 226 when moving to the right and slightly away from the mass 226when moving to the left. Among other things, this rotation of the massesreduces longitudinal stresses in the levers 228 and 230 and the primaryresonator flexures 240 and 244.

Likewise, the levers 232 and 234 essentially translate the substantiallyequal but opposite side-to-side motion of the masses 222 and 226 into asubstantially linear motion of the fork 238 along the y axis.Specifically, the side-to-side motion of the mass 222 is transferred tothe lever 232 through the pivot flexure 272, while the side-to-sidemotion of the mass 226 is transferred to the lever 234 through the pivotflexure 280. The levers 232 and 234 pivot at pivot flexures 274 and 282,respectively, and at the pivot flexures coupling the levers 232 and 234to the fork 238 (not shown), respectively, to cause the linear motion ofthe fork 238 along the y axis. These transfers cause the masses 222 and226 to rotate slightly as they move side-to-side. Specifically, the mass222 rotates slightly toward the mass 220 when moving to the left andslightly away from the mass 220 when moving to the right, while the mass226 rotates slightly toward the mass 224 when moving to the right andslightly away from the mass 224 when moving to the left. Among otherthings, this rotation of the masses reduces longitudinal stresses in thelevers 232 and 234 and the primary resonator flexures 242 and 246.

It should be noted that the symmetry of the resonator together with theprecision of the anti-phase motion causes the angular momenta from thepivoting motions to cancel and not induce rotation of the accelerometerframe.

FIG. 9 shows the relative movement of the masses 220, 222, 224, and 226and the forks 236 and 238. It should be noted that, in actuality, theseand other resonator structures move extremely small distances, and thearrows are greatly exaggerated to show that the masses 220, 222, 224,and 226 move side-to-side and rotate.

As discussed above, the masses are moved and controlledelectrostatically using electrostatic drivers. FIG. 11 shows a detailedview of an electrostatic driver, and, in particular, the electrostaticdriver 250 for mass 220. The electrostatic driver 250 is micromachinedso as to form a cavity within the mass 220 that includes two sets ofdrive fingers 1110 and 1120 that are integral to the mass 220 and twosets of electrode fingers 1130 and 1140 that are disposed within thecavity and are coupled to the substrate. The electrode fingers 1140 fitaround and between the drive fingers 1110, and the electrode fingers1130 fit around and between the drive fingers 1120. When a voltage isapplied to the electrode fingers 1140, the drive fingers 1110 are pulledtoward the electrode fingers 1140, generating a force on the mass 220toward the right. When a voltage is applied to the electrode fingers1130, the drive fingers 1120 are pulled toward the electrode fingers1130, generating a force on the mass 220 toward the left. Applyingvoltages alternately to the electrode fingers 1130 and to the electrodefingers 1140 causes the mass to move back and forth. The two sets ofelectrode fingers 1130 and 1140 are preferably anchored to the substratelinearly in order to reduce torque produced by surface shear of thesubstrate that can produce torque on the mass 220. It should be notedthat the electrostatic drivers 248, 252, 254, 256, 258, 260, and 262 aresubstantially the same as the electrostatic driver 250.

It should be noted that the electrostatic drivers 248, 252, 254, 256,258, 260, and 262 are positioned close to the middle of themicromachined gyroscope structure 100 so that most of the mass is awayfrom the center. This increases the sensitivity of the micromachinedgyroscope structure 100 to Coriolis accelerations.

There is also an electrostatic driver for the levers 228, 230, 232, and234. FIG. 8 shows a portion of the electrostatic driver 810 for thelevers 228, 230, 232, and 234. The electrostatic driver 810 ismicromachined so as to form a drive fingers on each lever and a set ofelectrode fingers that are coupled to the substrate. The electrodefingers fit around and between the drive fingers. When a voltage isapplied to the electrode fingers, the drive fingers are pulled towardthe electrode fingers, generating a force on each lever toward theelectrode fingers. The electrostatic driver 810 is used to reinforce themovement of the resonating structures. An alternative use for these isto sense the velocity of the resonator. That velocity signal can be usedto close an electromechanical oscillator loop which will excite theresonance.

It should be noted that the resonating structures are preferably drivenat or near their natural resonance frequency in order to enhance therange of motion of the resonating structures. This in turn increases thesensitivity of the gyroscope.

It should be noted that, in theory, the various gyroscope structures areperfectly balanced so that they move with substantially the samefrequency and phase. In practice, however, the various gyroscopestructures are not perfectly balanced. For example, the masses 220, 222,224, and 226 are theoretically identical (albeit mirror images in the xand/or y axes), but typically are not identical due at least in part tovariations in the material and processes used to form the masses.Similar imbalances can occur in other gyroscope structures, such as thevarious levers, pivots, and flexures. These imbalances can manifestthemselves in out-of-phase lateral movements of the masses (referredhereinafter to as “quadrature”), and can vary from device to device. Themechanical stiffnesses of the structures substantially suppresses thesemotions, but there is some residual quadrature.

Therefore, electrical quadrature suppression structures are typicallyused to reduce the amount of quadrature. The general principle wastaught by Clark in U.S. Pat. No. 5,992,233. In an embodiment of thepresent invention, a quadrature suppression structure typically includesat least one electrode located proximately to a portion of a mass alongthe direction of motion of the mass. When a voltage is applied to theelectrode, a resulting electrostatic force produces a lateral force thatattracts the mass toward the electrode. A single electrode is typicallyassociated with each mass, although not all electrodes are typicallyactivated. Rather, the quadrature behavior of a particular device istypically characterized to determine which (if any) electrodes toactivate to reduce the quadrature.

Because the amount of quadrature varies with the movement of the mass,it is preferable for the lateral force applied by the electrode tolikewise vary with the movement of the mass.

One way to vary the lateral force applied to the mass by the electrodeis to vary the voltage applied to the electrode based upon the positionof the mass. Specifically, the voltage would be increased as the massmoves outward toward the frame and would be decreased as the mass movesinward away from the frame. Such a solution would be very difficult inpractice.

Another way to vary the lateral force applied to the mass by theelectrode is to vary the amount of the mass that is adjacent to theelectrode based upon the position of the mass. FIG. 12 shows a detailedview of a quadrature suppression structure 1200 in accordance with anembodiment of the present invention. Two electrodes 1210 and 1220 areplaced between two adjacent masses 220 and 222, specifically in a cavityformed in and by the two masses 220 and 222. The electrode 1210 isadjacent to the mass 220, and is capable of applying a lateral force onthe mass 220 in the downward direction. The electrode 1220 is adjacentto the mass 222, and is capable of applying a lateral force on the mass222 in the upward direction. In order to vary the amount of lateralforce applied by an electrode, a notch is formed in each mass. The notchis formed adjacent to a portion of the electrode toward the end of theelectrode closer to the frame. As the mass moves outward toward theframe, the length of mass that is directly adjacent to the electrodeincreases, resulting in a larger lateral force applied to the mass. Asthe mass moves inward away from the frame, the length of mass that isdirectly adjacent to the electrode decreases, resulting in a smallerlateral force applied to the mass.

In a typical embodiment of the present invention, a voltage is appliedto one but not both of the electrodes 1210 and 1220. The electrode towhich a voltage is applied is typically selected by characterizing thequadrature and determining the electrode (if any) that most decreasesthe quadrature.

It should be noted that a similar quadrature suppression structure isformed between the masses 224 and 226. In order to cancel out staticforces, it is common to activate one electrode between the masses 220and 222 and one electrode between the masses 224 and 226.

It should be noted that the position of the quadrature suppressionelectrodes is not limited to a cavity at the juncture between twomasses. The electrodes can be placed in other positions. The positionsof the various electrodes should be balanced. The electrodes generallyproduce a certain amount of torque on the mass, and the amount of torquedepends at least to some degree on the position of the electrode. Asmall amount of torque is generally not a problem.

In a typical embodiment of the invention, a constant voltage is appliedto the electrode. This generally produces good results. Alternatively,the voltage applied to the electrode can be varied. When done properly,this can result in improved quadrature suppression, but at the cost ofincreased complexity.

Although FIGS. 1 and 2 show the accelerometer suspension flexures 202,204, 206, and 208 positioned at the four corners of the frame, it shouldbe noted that the present invention is not limited to such positioningof accelerometer suspension flexures. Rather, accelerometer suspensionflexures can be positioned at various points along the frame. Theaccelerometer suspension flexures preferably restrict translationalmovement of the frame while allowing rotational movement of the frameabout the center of mass. This can be accomplished by positioning theaccelerometer suspension flexures such that the linear axis between eachpair of opposing accelerometer suspension flexures passes through thegyroscope's effective center of mass.

In an alternate embodiment of the present invention, the accelerometersuspension flexures are placed at the middle of the four sides of theframe rather than at the four corners of the frame. FIG. 13 shows analternate frame suspension configuration in accordance with anembodiment of the present invention. In this embodiment, fouraccelerometer suspension flexures 1304, 1306, 1308, and 1310 are placedat the middle of the four sides of the frame 1302. There are certainproduction advantages to such a placement of the accelerometersuspension flexures. Specifically, certain etching equipment producesetches based upon a rectilinear grid, so it is easier to producefeatures that are aligned with the grid (as the side-positioned flexureswould be) compared to features that are set at an angle to the grid (asthe corner-positioned flexures would be). The corner-positioned flexuresare also not particularly space efficient.

The gyroscope is typically produced by depositing an oxide layer(approximately 2 um thick) on top of a substrate (approximately 600 umthick), using photolithography on the oxide layer to produce holes atdesired locations (and particularly at locations where the micromachinedgyroscope structure 100 is to be coupled to the substrate), depositing apolysilicon layer (approximately 4 um thick) over the oxide layer whichforms a thin film that bonds to the substrate through the holes in theoxide, using photolithography on the polysilicon layer to produce thecomplex structures of the micromachined gyroscope structure 100, andremoving the oxide layer using hydrofluoric acid. Thus, the resultingmicromachined gryoscope structure 100 is suspended approximately 2 umabove the substrate. It should be noted from the various drawings thatthe micromachined gyroscope structure 100 has a large number of holes,particularly in the masses 220, 222, 224, and 226, the levers 228, 230,232, and 234, and the frame 210. These holes are formed in themicromachined gyroscope structure 100 in order to allow the hydrofluoricacid to flow sufficiently through to the oxide layer. If such amicromachined gyroscope structure 100 was placed in a vacuum, themicromachined gyroscope structure 100 would typically be extremelyfragile and would typically have a high resonance frequency that tendsto ring. By operating the micromachined gyroscope structure 100 in air,the air cushions the micromachined gyroscope structure 100 and reducesringing.

It should be noted that a micromachined gyroscope of the presentinvention typically operates in air rather than a vacuum. Operation inair has a number of advantages and disadvantages. On one hand, air tendsto impede the motion of moving components due to viscous dampingresulting in smaller output signals, tends to give a phase shift thatspoils synchronous rectification, and tends to cause noise due to theimpact of air molecules (brownian motion) resulting in reducedsignal-to-noise ratio. On the other hand, however, operation in airenables the micromachined gyroscope to be a thin film structure,provides air cushioning that makes the thin film structure rugged, andeliminates the need for hermetic sealing of the gyroscope packageresulting in a lower overall cost of the final product.

The present invention may be embodied in other specific forms withoutdeparting from the true scope of the invention. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive.

Thus, the present invention is in no way limited to such things as theshape and size of the frame, the shape and size of the resonatingstructures (including masses, levers, forks, flexures, and pivotflexures), the number of movable masses, the manner in which theresonating structures are mechanically coupled, the number of fingersused for detecting Coriolis accelerations, the manner in which thecoriolis detectors are electrically coupled, the manner in which theresonating structures are driven, and the materials and manner in whichthe gyroscope is produced, among other things.

1. Apparatus comprising a plurality of resonator structures including apair of split masses suspended by suspension flexures, wherein eachsplit mass includes two lobes interconnected with a flexure that allowsthe lobes to rotate slightly as the masses resonate so as to reducelongitudinal stresses in the suspension flexures.
 2. Apparatus accordingto claim 1, wherein the pair of split masses are interconnected througha plurality of levers so as to resonate in anti-phase with one another,wherein the rotation of the lobes reduces longitudinal stresses in thelevers.
 3. Apparatus according to claim 2, wherein the plurality oflevers transform the coupled motion of the lobes from co-linear motionto parallel motion.
 4. Apparatus according to claim 1, wherein each lobeincludes a plurality of drive fingers interdigitated with acorresponding array of fixed drive fingers affixed to a substrate. 5.Apparatus according to claim 1, wherein each lobe comprises at least onenotch for electronic quadrature suppression.
 6. Apparatus according toclaim 2, wherein the plurality of resonator structures are suspendedwithin an inner perimeter of a frame and wherein the resonatorstructures are mechanically coupled to produce substantially a singleresonance frequency so as to restrict transfer of inertial forces to theframe.
 7. Apparatus according to claim 6, wherein each of the pluralityof levers is coupled at one end to the frame and at another end to adifferent one of the lobes, and wherein each lever has pivots, definedat the points of attachment to the frame and the lobe by theintersection of the axes of at least two orthogonal flexures, to ensurethat the attachment point cannot translate with respect to the lever. 8.Apparatus according to claim 2, wherein each of the levers includes aplurality of lever fingers interdigitated with corresponding fixedfingers affixed to an underlying substrate for at least one of drivingthe lever and sensing position of the lever.
 9. Apparatus according toclaim 8, wherein each of the levers moves with an arcuate motion, andwherein the lever fingers are disposed at varying angles so as tomaintain substantially equal distances from said corresponding fixedfingers during movement of the levers.
 10. Apparatus according to claim1, wherein the plurality of resonator structures are micromachined froma single wafer.
 11. Apparatus comprising a plurality of resonatorstructures including a first pair of masses coupled through a firstflexure and a second pair of masses coupled through a second flexure,each mass suspended by a suspension flexure, wherein the first andsecond flexures allow the masses to rotate slightly as the massesresonate so as to reduce longitudinal stresses in the suspensionflexures.
 12. Apparatus according to claim 11, wherein the first andsecond pairs of masses are interconnected through a plurality of leversso as to resonate in anti-phase with one another, wherein the rotationof the masses reduces longitudinal stresses in the levers.
 13. Apparatusaccording to claim 12, wherein the plurality of levers transform thecoupled motion of the masses from co-linear motion to parallel motion.14. Apparatus according to claim 11, wherein each mass includes aplurality of drive fingers interdigitated with a corresponding array offixed drive fingers affixed to a substrate.
 15. Apparatus according toclaim 11, wherein each mass comprises at least one notch for electronicquadrature suppression.
 16. Apparatus according to claim 12, wherein theplurality of resonator structures are suspended within an innerperimeter of a frame and wherein the resonator structures aremechanically coupled to produce substantially a single resonancefrequency so as to restrict transfer of inertial forces to the frame.17. Apparatus according to claim 16, wherein each of the plurality oflevers is coupled at one end to the frame and at another end to adifferent one of the masses, and wherein each lever has pivots, definedat the points of attachment to the frame and the mass by theintersection of the axes of at least two orthogonal flexures, to ensurethat the attachment point cannot translate with respect to the lever.18. Apparatus according to claim 12, wherein each of the levers includesa plurality of lever fingers interdigitated with corresponding fixedfingers affixed to an underlying substrate for at least one of drivingthe lever and sensing position of the lever.
 19. Apparatus according toclaim 18, wherein each of the levers moves with an arcuate motion, andwherein the lever fingers are disposed at varying angles so as tomaintain substantially equal distances from said corresponding fixedfingers during movement of the levers.
 20. Apparatus according to claim11, wherein the plurality of resonator structures are micromachined froma single wafer.